Handle runway damage in the debrief.

Apparently we were already getting this info because it's a unit like
any other according to the event system, so if runways were actually
sufficiently damaged we'd emit a ton of exceptions.

This doesn't do anything yet, but tracks the damage state. Will add the
ability to repair them next, and then finally make the damage render the
runway inoperable.

game/unitmap.py

@@ -3,12 +3,14 @@ from typing import Dict, Optional
 
 from dcs.unitgroup import FlyingGroup
 
+from game.theater import Airfield
 from gen.flights.flight import Flight
 
 
 class UnitMap:
     def __init__(self) -> None:
         self.aircraft: Dict[str, Flight] = {}
+        self.airfields: Dict[str, Airfield] = {}
 
     def add_aircraft(self, group: FlyingGroup, flight: Flight) -> None:
         for unit in group.units:
@@ -21,3 +23,11 @@ class UnitMap:
 
     def flight(self, unit_name: str) -> Optional[Flight]:
         return self.aircraft.get(unit_name, None)
+
+    def add_airfield(self, airfield: Airfield) -> None:
+        if airfield.name in self.airfields:
+            raise RuntimeError(f"Duplicate airfield: {airfield.name}")
+        self.airfields[airfield.name] = airfield
+
+    def airfield(self, name: str) -> Optional[Airfield]:
+        return self.airfields.get(name, None)